Using time-resolved penumbral imaging to measure low x-ray emission signals from capsule implosions at the NIF

We have developed an experimental platform at the National Ignition Facility to measure x-ray Thomson scattering (XRTS) spectra from indirectly-driven capsule implosions that create extreme density conditions near stagnation [D. Kraus et al, J. Phys. Conf. Series 717, 012067 (2016).]. In order to account for shot-to-shot variations of the stagnation time and to benchmark the achieved plasma conditions between shots and against radiation hydrodynamics simulations, we need to know the relative timing between the scattering measurement and the peak x-ray emission, which occurs at stagnation. Due to lower implosion velocity, use of a low gas fill capsule, and hot spot symmetry perturbations, the hot spot emission is 100 - 1000x weaker than that of standard ICF implosions. To address this challenge, we have developed and fielded a new pinhole-imaging snout that exploits time-resolved penumbral imaging. Using 150 µm diameter penumbral-imaging pinholes, a time series of 2D images can be reconstructed through analysis of the penumbras. The reconstructions allow us to extract the spatially and temporally resolved evolution and timing of the implosion through stagnation. We use differential filtering to extract plasma temperatures, additionally constraining the thermophysical plasma conditions. Despite fluctuations of the x-ray flash intensity of up to 5x, the emission time history is similar from shot to shot, and slightly asymmetric with respect to peak x-ray emission. Peak emission times vary by up to 250 ps and can be determined with an accuracy of 50 ps.